n the form of a rhombic
dodecahedron, or as a trisoctahedron (called also sometimes an
icosatetrahedron), or a mixture of the two, though the stones appear in
other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They
average from 40 to about 42 per cent. of silica, the other ingredients
being in fairly constant and definite proportions. They are vitreous and
resinous in their lustre and of great variety of colour, chiefly amongst
reds, purples, violets, greens, yellows and blacks, according to the
colouring matter present in their mass. There are many varieties which
are named in accordance with one or more of their constituents, the best
known being: (A) The iron-alumina garnet, having the formula 6FeO,
3SiO_{2} + 2Al_{2}O_{3}, 3SiO_{2}. This is the "precious" garnet, or
almandine, sometimes called the "Oriental" garnet; these stones are
found in Great Britain, India, and South America, and are deep red and
transparent, of vitreous lustre. They get up well, but certain varieties
are so subject to defects in their substance, brought about by pressure,
volcanic action, and other causes, some of which are not yet known, that
their quality often becomes much depreciated in consequence. This
inferior variety of the iron-alumina garnet is called the "common"
garnet, and has little lustre, being sometimes opaque. The perfect
qualities, or almandine, as described above, are favourite stones with
jewellers, who mount great quantities of them.

The second variety is the (B) lime-iron garnet, formula, 6CaO,3SiO_{2} +
2Fe_{2}O_{3},3SiO_{2}. The chief of this class is the melanite,
sometimes dull, yet often vitreous; it is mostly found in volcanic
rocks, such as tuff; this variety is very popular with jewellers for
mourning ornaments, for as it is a beautiful velvet-black in colour and
quite opaque, it is pre-eminent for this purpose, being considerably
less brittle than jet, though heavier. Another variety is the
"topazolite," both yellow and green. The "aplome" is greenish-yellow,
yellowish-green, brown, and usually opaque. A further form of lime-iron
garnet is the "pyreneite," first found in the Pyrenees Mountains, hence
its name.

The (C) lime-chrome garnets--6CaO,3SiO_{2} + 2Cr_{2}O_{3}, 3SiO_{2}--the
chief of which is "uwarowite." This is of a magnificent emerald green
colour, translucent at edges and of a vitreous lustre. When heated on
the borax bead it gives an equally beautiful green, which is, however,
rather